Cancer remains one of the major health problems associated with aging, yet the role of the aging process in cancer remains to be determined. One approach to this problem is to study aging at the molecular level using cellular models of aging, which may provide insights into both the cancer and the aging processes. We have shown that defects in the senescence program in cancer cells can be corrected by introduction of human chromosome 1 from normal cells into immortal cells. Cellular senescence is a state of irreversible cell cycle arrest in which normal cells fail to enter into DNA synthesis upon serum stimulation. We examined whether proteins required for G1/S cell cycle progression were irreversibly down- regulated in senescent human fibroblasts. In contrast to young cells where both forms of MAP-kinase were phosphorylated on tyrosine in response to serum, the p42MAP-kinase was not tyrosine phosphorylated upon serum stimulation, whereas p44MAP-kinase was phosphorylated on tyrosine in serum- starved or serum-stimulated senescent cells. Cdc2 and cyclin A mRNAs were completely down-regulated in senescent fibroblasts. Clones expressing the transfected human cyclin A or cdc2 genes senesced at a population doubling similar to controls. However, significant extension of life span were seen in cells that expressed both the transfected human cyclin A and cdc2 genes, suggesting that these two proteins may be important in controlling the life span of cells. We investigated the possible role of phosphatases in senescence. Treatment of quiescent hamster and human fibroblasts with phosphatase inhibitors (sodium orthovanadate or okadaic acid) allowed cells to progress from Go/G1 arrest to S-phase. In phosphatase inhibitor-treated quiescent Syrian hamster embryo fibroblasts, phosphorylation of RB and MAP- kinase proteins and induction of cdc2 protein accompanied this progression to DNA synthesis, similar to the effects of serum or mitogen treatment. Phosphatase inhibitors could also override the block to DNA synthesis in senescent cells. This suggests that protein phosphatases may play a role in the negative regulation of cell growth and maintenance of growth arrest.